1. Field of the Invention
The invention relates to a process for electronic processing of multicolored continuous-tone (contone) images, during which process the original, divided in the shape of an orthogonal screen of areal scan elements, is scanned line by line or column by column; the brightness of the individual scan elements is determined and converted into digital tone values, the brightness being based on the respective color separations; a screened reproduction of the original that is divided into square recording fields of expediently identical size is obtained by printing a number of dots, which correspond to the respective tone value, exhibit identical color density, and are set on the individual areal recording elements of the respective recording field; each recording field is divided into square tiles of expediently identical size with a specific number of recording elements; the number of tone levels that can be differentiated within a tile is specified by the number of recording elements contained in a tile; distributions (dot pattern) of the dots corresponding numerically to the tone levels are assigned to the individual tone levels within the tile, distributions being defined within the tile and determined prior to the completion of screening and held available in a data memory (as bit maps); and the specific dot patterns are obtained, starting from fixed basic dot patterns, by adding or subtracting dots.
The term "dots" is understood to denote any kind of graphic dot representation, where as the term "dot" is not limited to a geometric point but rather represents an area element of small size.
2. Description of the Related Art
With such a process multicolored continuous-tone images can be reproduced on a recording medium, e.g. a sheet of paper, a printing plate or a film by means of individual recording elements, which are of identical size and whose total area is either inked up or is not inked up. The image information of the continuous-tone image is first scanned by a sensor or scanning device, such as a scanner. To this end, the continuous-tone image is scanned, e.g., line by line, scan element by scan element, with a light beam. The scanner emits for each scan element an analog voltage signal, which corresponds to the brightness of the scan element illuminated by incident light. In the case of colored originals, the light beam is dispersed into three colors (red, green, blue) by way of a filter and the brightness of each is determined. For the purpose of increasing the local contrast (so-called detail contrast) the ambient field of the scan element is frequently also recorded. The analog voltage signals are subdivided by way of analog-digital converters into digital tone values of, e.g., 256 (=2.sup.8) tone levels (0-255), so that they can be processed as 8 bit data blocks.
Continuous-tone images, where the tone values lie between a minimum and a maximum value, are recorded as screened halftone copies, where dots with maximum tone value are applied on the recording medium. The tone value determined for a specific spot of the continuous-tone image is simulated by putting a number of dots, which result in a percentage dot area corresponding to the tone value, on the corresponding spot of the screened reproduction. One or several dots can be combined into a screen dot, so that, e.g. in the individual recording fields, which comprise recording elements and into which the halftoned copy is subdivided, each screen dot comprises a more or less large number of dots. Thus, it is possible to reproduce the original picture with numerous discrete dots, i.e. halftoned.
In the conventional screening method to reproduce the tone value at one spot of the original, a number of dots that correspond to the percentage dot area are combined into a single screen dot, which is put in the center of the recording field. When overprinting several color separations, the resulting color perceived by the eye depends on the extent to which the screen dots of the individual color separations overlap. Small, mounting-induced displacements of the color separations (socalled register variations) can lead to changes in color. With the conventional screen this disturbing effect is counteracted through angular rotation of the screen of the individual color separations. In so doing, however, the problem of forming rosetta moires and other moire patterns arises and the number of possible color separations is limited. In this manner and with other disturbing effects such as tone skip when dot contact is occurring for the first time and obliteration of dots at low tone values--the quality of the halftoned copy is impaired. In addition, recording moires can occur when in a color separation there is interference between the screen and regular (periodic) structures of the original.
To avoid such disturbing effects a number of screening methods with random dot distribution have been developed (cf. e.g. V. Humbel "Frequency modulated screening and its suitability for low resolution reproduction systems", Report 1989/1, EMPA, UGRA, St. Gallon, December 1990), in which process the dots to be set are not combined in the center of the recording field, but rather are distributed more or less arbitrarily (randomly) over the entire recording field. It has been demonstrated, however, that when dots are distributed randomly a certain granularity and excessive noise in the screened reproduction is produced, a feature that occurs in particular in the midtone range, when in many places--nonuniformly distributed--dot contact occurs for the first time between several dots. In addition, as a consequence of randomly setting the dots even in areas with identical tone value accumulations of dots and areas with below average dot number can occur and lead to darker or brighter spots (formation of clustered dots and clustered no prints).